Low-magnitude Seismicity with a Downhole Distributed Acoustic Sensing Array -- examples from the FORGE Geothermal Experiment

TL;DR

This study demonstrates the effectiveness of downhole Distributed Acoustic Sensing arrays in detecting and characterizing low-magnitude earthquakes near the FORGE geothermal site, outperforming surface arrays in sensitivity.

Contribution

It introduces a novel application of DAS technology for earthquake monitoring in geothermal settings, showing improved detection capabilities and potential for enhanced event localization.

Findings

DAS array detected 82 earthquakes in 10.5 days.

DAS processing achieved at least 0.5 magnitude better completeness than surface arrays.

Multiple DAS wells can help resolve event locations despite azimuthal ambiguity.

Abstract

We show the capabilities of a downhole Distributed Acoustic Sensing (DAS) array in detecting, locating and characterizing low-magnitude earthquakes occurring in the vicinity of the Frontier Observatory for Research in Geothermal Energy (FORGE) site in Utah. 10.5 days of continuous data were acquired in a monitoring well at the FORGE geothermal site during the initial stimulation of an Enhanced Geothermal System in April-May 2019. Earthquake activity beneath Mineral Mountains, Utah also occurred within 10 km of the FORGE monitoring well. During the experiment, four events from those areas were cataloged by the University of Utah Seismograph Stations. Our processing of DAS data, including template matching, finds 82 earthquakes during that period, of which 16 are visible on the regional network. The magnitude of completeness obtained by DAS processing is better by at least M=0.5 than the dense surface array around the FORGE site. While a single vertical DAS array is limited in terms of event location due to its azimuthal ambiguity, multiple DAS wells or a combination of a downhole array with surface stations or near-surface horizontal DAS could jointly resolve locations. All detected events probably originated from the two active source areas and can be clustered into several distinct families.